The production of light olefins (ethylene, propylene and butenes) from various hydrocarbon feedstocks utilizes the technique of pyrolysis, or steam cracking Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules.
In the steam cracking process, it is desirable to maximize the recovery of useful heat from the process effluent stream exiting the cracking furnace. Effective recovery of this heat is one of the key elements of a steam cracker's energy efficiency.
The steam cracking process, however, also produces molecules which tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that, under certain conditions, can foul heat exchange equipment, rendering heat exchangers ineffective. The fouling propensity can be characterized by three temperature regimes.
Above the hydrocarbon dew point (the temperature at which the first drop of liquid condenses), the fouling tendency is relatively low. Vapor phase fouling is generally not severe, and there is no liquid present that could cause fouling. Appropriately designed transfer line heat exchangers are therefore capable of recovering heat in this regime with minimal fouling.
Between the hydrocarbon dew point and the temperature at which steam cracked tar is fully condensed, the fouling tendency is high. In this regime, the heaviest components in the stream condense. These components are believed to be sticky and/or viscous, causing them to adhere to surfaces. Furthermore, once this material adheres to a surface, it is subject to thermal degradation that hardens it and makes it more difficult to remove.
At or below the temperature at which steam cracked tar is fully condensed, the fouling tendency is relatively low. In this regime, the condensed material is fluid enough to flow readily at the conditions of the process, and fouling is generally not a serious problem.
One technique used to cool pyrolysis unit effluent and remove the resulting tar employs heat exchangers followed by a water quench tower in which the condensibles are removed. This technique has proven effective when cracking light gases, primarily ethane, propane and butane, because crackers that process light feeds, collectively referred to as gas crackers, produce relatively small quantities of tar. As a result, heat exchangers can efficiently recover most of the valuable heat without fouling and the relatively small amount of tar can be separated from the water quench albeit with some difficulty.
This technique is, however, not satisfactory for use with steam crackers that crack naphthas or feedstocks heavier than naphthas, collectively referred to as liquid crackers, since liquid crackers generate much larger quantities of tar than gas crackers. Heat exchangers can be used to remove some of the heat from liquid cracking, but only down to the temperature at which tar begins to condense. Below this temperature, conventional heat exchangers cannot be used because they would foul rapidly from accumulation and thermal degradation of tar on the heat exchanger surfaces. In addition, when the pyrolysis effluent from these feedstocks is quenched, some of the heavy oils and tars produced have approximately the same density as water and can form stable oil/water emulsions. Moreover, the larger quantity of heavy oils and tars produced by liquid cracking would render water quench operations ineffective, making it difficult to raise steam from the condensed water and to dispose of excess quench water and the heavy oil and tar in an environmentally acceptable manner.
Accordingly, in most commercial liquid crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator, and a water quench tower or indirect condenser. For a typical heavier than naphtha feedstock, the transfer line heat exchangers cool the process stream to about 593° C. (1100° F.), efficiently generating super-high pressure steam which can be used elsewhere in the process. The primary fractionator is normally used to condense and separate the tar from the lighter liquid fraction, known as pyrolysis gasoline, and to recover the heat between about 93° and about 316° C. (200° F. to 600° F.). The water quench tower or indirect condenser further cools the gas stream exiting the primary fractionator to about 40° C. (100° F.) to condense the bulk of the dilution steam present and to separate pyrolysis gasoline from the gaseous olefinic product, which is then sent to a compressor.
Modern quench systems for cooling hot pyrolysis effluent typically employ at least some indirect heat exchange in which furnace effluent is cooled in a heat-exchanger where high pressure boiler feed water is vaporized to produce high pressure steam. High pressure boiler feed water is obtained from a deaerator and is typically provided at pressures ranging from about 4240 to about 13900 kPa (600 to 2000 psig) and temperatures ranging from about 100° C. to about 260° C. (212 to 500° F.). Typical steam pressure levels employed range from about 4240 to about 13893 kPa (600 to 2000 psig). The steam generated in the quench exchangers is typically superheated in the convection section of an associated steam cracking furnace, and the superheated steam is used within the ethylene plant to power large steam turbines that can drive, e.g., major compressors or pumps.
In currently employed quench systems, energy recovered from the heated process gas is limited. As the furnace effluent stream cools, it eventually reaches its dew point, the temperature at which the heaviest cracking by-product components begin to condense, forming materials known as tar, pitch, or non-volatiles, from their precursors present in the furnace effluent stream. Such materials are still highly reactive at the temperatures at which they first condense. When deposited against a relatively hot surface, e.g., a quench exchanger tube wall, these materials continue to cross-link, polymerize and/or dehydrogenate to form an undesirable highly insulating foulant or coke layer on such surface. Yields of tar, pitch or non-volatile components generated in a cracking furnace generally increase as molecular weight of feed to the furnace increases, although the molecular structure of heavy feeds also can influence tar yield. For example, a heavy, highly paraffinic feed may have a lower tar yield than a lighter feed of lower paraffin content, but higher naphthene and/or aromatics content.
Dew point, or the temperature at which condensate is initially formed, of a gaseous effluent from pyrolysis typically increases as the yield of heavy tar components increase. Thus, effluent dew point generally increases as the feed molecular weight increases. Typical effluent dew points are as follows: for ethane cracking, about 149° C. (300° F.), for light virgin naphtha cracking, from about 287° to about 343° C. (550 to 650° F.), for gas oil cracking, from about 399° to about 510° C. (750° to 950° F.), and for vacuum gas oil (VGO) cracking, up to about 566° C. (1050° F.).
Conventional quench exchanger trains are designed to keep the process-side wall temperature, i.e., the exchanger surface in contact with the process gas effluent, at or above the effluent dew point.
Thus, ethane quench systems typically employ steam generating heat exchangers operating at from about 4240 kPa to about 10445 kPa (600 to 1500 psig), with corresponding process-side wall temperatures in the range of from about 253° to about 316° C. (488° to 600° F.). These steam generating quench exchangers cool the furnace effluent to a temperature of about 288° to about 343° C. (550° to 650° F.). Further energy recovery from the furnace effluent can be effected by preheating the boiler feed water supply to the steam generating system, thus further increasing the overall cycle efficiency. So long as the process-side wall temperatures of the high pressure boiler feed water (HPBFW) preheater are maintained above the dew point, fouling is negligible. Thus, ethane furnace effluent can be efficiently quenched and cooled down to about 204° C. (400° F.) without fouling problems.
Modern naphtha furnaces typically employ quench exchangers generating steam at pressures from about 10445 to about 13890 kPa (1500 to 2000 psig). Effluent is typically cooled to a temperature ranging from about 343° to about 399° C. (650° to 750° F.) with negligible fouling occurring as the film temperatures on the process-side heat exchanger surfaces are kept at or above the effluent dew point. However, further cooling in high pressure boiler feed water (HPBFW) preheaters is not practiced because of associated fouling below the dew point. If further cooling is required, a cooling liquid quench medium, e.g., quench oil or water, can be directly injected to achieve the desired temperature without fouling.
For modern gas oil furnaces associated with hydrocarbon pyrolysis, a quench heat exchanger generating steam at pressures from about 10445 to about 13890 kPa (1500 to 2000 psig) can be used. Clean heat exchanger outlet temperatures typically range from about 427° to about 482° C. (800° to about 900° F.), but the exchanger fouls rapidly until the foulant/process gas interface temperature reaches the effluent dew point, at which stage fouling rates slow dramatically. At the end of a typical gas oil run, the heat exchangers will have reached effluent outlet temperatures ranging from about 538° to about 677° C. (1000° to about 1250° F.).
Inasmuch as effluent from a gas oil furnace must be cooled to a temperature ranging from about 287° to about 316° C. (550° to about 600° F.), a liquid quench oil stream is typically mixed with the heat exchanger effluent to achieve such cooling. The heat absorbed by the quench oil can be recovered in a fractionator pump around circuit. However, the relatively low temperature of the pumparound stream, less than about 287° C. (550° F.), yields only medium pressure steam, typically, from about 790 to 1830 kPa (100 to 250 psig) or low pressure steam below about 790 kPa (100 psig). This represents a significant efficiency reduction compared to the generation of high pressure steam, e.g., about 10445 kPa (1500 psig), achieved by furnaces using ethane or other gaseous feedstocks.
The present invention seeks to provide a simplified method for treating pyrolysis unit effluent, particularly the effluent from the steam cracking of hydrocarbonaceous feeds that are heavier than naphthas. Heavy feed cracking is often more economically advantageous than naphtha cracking, but in the past it suffered from poor energy efficiency and higher investment requirements. The present invention optimizes recovery of the useful heat energy resulting from heavy feed steam cracking without fouling of the cooling equipment. This invention can also obviate the need for a conventional primary fractionator tower and its ancillary equipment.
Heavy feed steam cracking effluent can be treated by using a primary heat exchanger, typically a transfer line exchanger, generating high pressure steam to initially cool the furnace effluent. The surfaces of heat exchanger tubes must operate above the hydrocarbon dew point to avoid rapid fouling, typically an average bulk outlet temperature of about 593° C. (about 1100° F.) for a heavy gas oil feedstock. Additional cooling can be provided by directly injecting a quench liquid such as tar or distillate to immediately cool the stream without fouling. Alternatively, the pyrolysis furnace effluent can be directly quenched, e.g., with distillate, which also avoids fouling. However, the former cooling method suffers from the drawback that only a fraction of the heat is recovered in a primary transfer line exchanger; moreover, in both methods, remaining heat removed by direct quenching is recovered at a lower temperature where it is less valuable. Furthermore, additional investment is required in the downstream primary fractionator where low level heat is ultimately removed, and in offsite boilers which must generate the remaining high pressure steam required by the steam cracking plant.
Relevant background art is discussed below.
“Latest Developments in Transfer Line Exchanger Design for Ethylene Plants”, H. Herrmann & W. Burghardt, Schmidt'sche Heissdampf-Gesellschaft, prepared for presentation at AIChE Spring National Meeting, Atlanta, April 1994, Paper #23c, as well as U.S. Pat. No. 4,107,226, disclose dew point fouling mechanisms in ethylene furnace quench systems, as well as use of heat exchangers which generate high pressure steam.
U.S. Pat. Nos. 4,279,733 and 4,279,734 propose cracking methods using a quencher, indirect heat exchanger and fractionator to cool effluent, resulting from steam cracking. The latter reference teaches a method utilizing a first stage “dry-wall” quench exchanger cooling the hot process effluent to at least 540° C. (1000° F.) wherein liquid washed quench exchangers recover energy to high pressure steam at temperatures below the dew point of the effluent gas stream.
U.S. Pat. Nos. 4,150,716 and 4,233,137 propose a heat recovery apparatus comprising a pre-cooling zone where the effluent resulting from steam cracking is brought into contact with a sprayed quenching oil, a heat recovery zone and a separating zone. The latter reference teaches a method utilizing liquid washed quench exchangers to recover energy to high pressure steam at temperatures below the dew point of the effluent gas stream, wherein energy recovery to high pressure steam is achievable at 250° to 300° C. (482° to 572° F.), with substantial precooling of the hot effluent to 300° to 400° C. (572° to 752° F.), requiring a high circulation rate of quench, e.g., up to 21:1 quench to hydrocarbon feed, requiring a substantial investment in circulation pumps and pipework as well as associated energy consumption.
U.S. Pat. No. 4,614,229 discloses heat recovery from hot effluent using a primary transfer line exchanger and a secondary transfer line exchanger utilizing wash liquid injected into its tubes to provide process gas cooled to about 550° F. Energy recovery at lower temperature is carried out in a fractionator pumparound circuit, favoring recovery of steam at medium pressures. Liquid collected from the secondary TLE for use as a wash liquid increases the concentration of undesirable heavy, viscous molecules, increasing the effluent dew point and fouling tendencies. Liquid washing of exchanger tubes relies upon uniform flow patterns across the exchanger inlet tubesheet/baffle, which technique is susceptible to degradation of uniform wash liquid distribution over time.
Lohr et al., “Steam-cracker Economy Keyed to Quenching,” Oil Gas J., Vol. 76 (No. 20) pp. 63-68 (1978), proposes a two-stage quenching involving indirect quenching with a transfer line heat exchanger to produce high-pressure steam along with direct quenching with a quench oil to produce medium-pressure steam.
U.S. Pat. Nos. 5,092,981 and 5,324,486 propose a two stage quench process for effluent from steam cracking, comprising a primary transfer line exchanger which functions to rapidly cool furnace effluent and to generate high temperature steam and a secondary transfer line exchanger which functions to cool the furnace effluent to as low a temperature as possible consistent with efficient primary fractionator or quench tower performance and to generate medium to low pressure steam.
U.S. Pat. No. 5,107,921 proposes transfer line exchangers having multiple tube passes of different tube diameters. U.S. Pat. No. 4,457,364 proposes a close-coupled transfer line heat exchanger unit.
U.S. Pat. No. 3,923,921 proposes a naphtha steam cracking process comprising passing effluent through a transfer line exchanger to cool the effluent and thereafter through a quench tower.
WO 93/12200 proposes a method for quenching the gaseous effluent from a hydrocarbon pyrolysis unit by passing the effluent through transfer line exchangers and then quenching the effluent with liquid water so that the effluent is cooled to a temperature in the range of 105° C. to 130° C. (221° F. to 266° F.), such that heavy oils and tars condense, as the effluent enters a primary separation vessel. The condensed oils and tars are separated from the gaseous effluent in the primary separation vessel and the remaining gaseous effluent is passed to a quench tower where the temperature of the effluent is reduced to a level at which the effluent is chemically stable.
EP 205 205 proposes a method for cooling a fluid such as a cracked reaction product by using transfer line exchangers having two or more separate heat exchanging sections.
JP 2001040366 proposes cooling mixed gas in a high temperature range with a horizontal heat exchanger and then with a vertical heat exchanger having its heat exchange planes installed in the vertical direction. A heavy component condensed in the vertical exchanger is thereafter separated by distillation at downstream refining steps.
WO 00/56841, GB 1,390,382, GB 1,309,309, U.S. Pat. Nos. 4,444,697; 4,446,003; 4,121,908; 4,150,716; 4,233,137; 3,923,921; 3,907,661; and 3,959,420; propose various apparatus for quenching a hot cracked gaseous stream wherein the hot gaseous stream is passed through a quench pipe or quench tube wherein a liquid coolant (quench oil) is injected.
U.S. Pat. Nos. 4,107,226; 3,593,968; 3,907,661; 3,647,907; 4,444,697; 3,959,420; 4,121,908; and 6,626,424; and Great Britain Patent Application 1,233,795 disclose methods of distributing wash liquids in quench fittings, e.g., annular direct quench fittings.
Given the foregoing, it would be desirable to recover useful heat from steam cracking furnace effluent in the absence of rapid fouling and absent direct quenching in order to minimize overall energy consumption in steam cracking processes used to manufacture light olefins.